1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same, which are suitable for an image forming apparatus using an electrophotography process such as a color laser beam printer, a color digital copying machine, or a color multifunction printer.
2. Description of the Related Art
A color image forming apparatus such as a color laser printer in recent years needs to satisfy high demands for low cost, high speed, and high image quality. As a method of realizing high speed of the color image forming apparatus, there is widely known a tandem method in which an image bearing member disposed for each color separately is scanned by a light beam so that an image for each color is formed, and later the plurality of images are superposed on a transferring medium so as to form a color image.
In addition, for realizing high speed printing, a light source emitting a plurality of light beams may be used. In this case, if polarization directions of the light beams are not uniform, a ratio between P polarization component and S polarization component (polarization component ratio) for an optical element that transmits or reflects the light beams before reaching a photosensitive member as the image bearing member is different among the light beams. As a result, a light amount at each image height on a surface to be scanned (illuminance distribution) is different among the light beams.
In order to solve this problem, there is known a scanning optical apparatus in which P polarization reflectance and S polarization reflectance are set to be substantially the same for the optical element that transmits or reflects the light beams. Thus, even in the case where the polarization component ratio changes from a design value, unevenness in illuminance distribution on the surface to be scanned is decreased.
For instance, Japanese Patent Application Laid-Open No. H11-326807 discloses an example in which a linear polarization direction of a laser is adjusted for correcting shading (illuminance distribution) on an image bearing member. Japanese Patent Application Laid-Open No. 2001-337285 discloses an example in which reflectances for S-polarized and P-polarized lights are set to be substantially the same as each other so that an illuminance distribution becomes uniform.
However, in the conventional technology described above in Japanese Patent Application Laid-Open No. H11-326807, it is necessary to adjust the polarization direction of each of the lasers while checking the illuminance distribution to correct the illuminance distribution. Therefore, as the number of light sources increases, assembly cost in the adjustment increases. In addition, in the conventional technology described above in Japanese Patent Application Laid-Open No. 2001-337285, it is difficult to set exactly the same reflectance for the P-polarized light and for the S-polarized light.